1. Field of the Invention
Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC), is a Gram-negative bacterium that is highly infectious for the channel catfish Ictalurus punctatus. This pathogen has been isolated from channel catfish in most areas of the United States where this species is cultured and from walking catfish in Thailand [Kasornchandra et al., J. Fish Dis. 10:137-138(1987). Losses in fingerlings to market-size fish account for about 50% of the total disease losses to catfish producers. Formalin-killed bacterial vaccines have not been successful due to their provoking only a humoral response. This is due to Edwardsiella ictaluri being an intracellular pathogen, with acquired immunity in fish surviving ESC being more cellular than humoral.
This invention relates to a novel vaccine against ESC which does in fact provide superior protection over existing commercial vaccines due to its ability to stimulate cellular-immunity and protection.
2. Description of the Prior Art
It has been estimated that ESC is responsible for annual losses of 20 to 30 million dollars in the catfish industry. Control of E. ictaluri has been attempted by feeding antibiotic medicated feed. This expensive practice has been ineffective because sick fish do not eat and antibiotic resistance to oxytetracycline and ormethoprim-sulfamethoxine, the approved drugs for use on food fish, has been observed.
Based on the success of disease control by bacterin immunization of salmonids, bacterins were developed against ESC. An E. ictaluri bacterin (ESC vaccine, ALPHARMA, formerly BIOMED, Inc., 1720-130th Ave. N.W., Bellevue, Wash., USA) was produced and marketed with a provisional license for immersion vaccination of channel catfish against ESC. An oral E. ictaluri bacterin (Escogen, Aqua Health, Ltd., Charlottetown, PEI, Canada) was produced, marketed and licensed for oral vaccination of channel catfish against ESC. Thune et al. [J. Appl. Aquaculture 3, 11-23 (1994)] teach that vaccination of channel catfish (Ictalurus punctatus) with E. ictaluri bacterins has not resulted in acquired immunity of long duration. Nusbaum and Morrison [J. Aquat. Animal Health 8, 146-149 (1996)] also demonstrated that killed E. ictaluri was not entering the fish and suggested that this affected vaccine efficacy. Edwardsiella ictaluri has been described as a facultative intracellular pathogen of channel catfish [Miyazaki et al.; J. Fish Dis. 8, 389-392 (1985)]. Because of the nature of E. ictaluri as an intracellular pathogen, it is not surprising that killed vaccines have not been successful. Strictly controlled live vaccination of channel catfish with low concentrations of unmodified isolates of E. ictaluri by bath immersion was found to stimulate protective immunity, specific antibodies by Klesius et al. [Journal of Aquatic Animal Health 7, 205-210 (1995)]. Protective immunity to B. abortus in cattle was demonstrated by Montaraz et al. [Infect. Immun. 53, 245-251 (1986)] and Schurig et al. [Vet. Micro. 28, 171-188 (1991)] using vaccines based on modified live rifampicin-resistant Brucella species.